Model-based performance monitoring requires comparison of data generated from two distinct kinds of system, the Building Energy Model (BEM) and the Building Automation System (BAS). The HVAC design process at present places no requirement for interoperability between them. In the performance monitoring use-case, they will no longer be stand-alone elements. ASHRAE Guideline 20-2010 Documenting HVAC&R Work Processes for Data Exchange Requirements provides a use-case approach to information issues. The Guideline 20 process is applied to BEM-BAS data comparisons to better understand where incompatibilities lie. Examples are developed such as equipment naming, operating profiles, energy and physical parameters.